The anodes include a conducting metal stem, in conjunction with a device for fixing to said superstructure and with an electrical connection device, and a block made of carbonaceous material which constitutes the body of the anode and which is introduced into the electrolyte bath. The connection between the anode stem and the body made of carbonaceous material is made via a leg, typically made of steel, integral with the base of the stem and which in general has the shape of an upside-down candelabrum, each branch of the candelabrum being associated with a cylindrical end whose axis is parallel to the stem and known as a “stub”. Generally speaking, these stubs are inserted inside cavities made on the top face of the block of carbonaceous material and the gaps between the stubs and the bores are filled with molten metal, typically cast iron. The metal bushings thus formed—also referred to as “thimbles”—make it possible to ensure good mechanical attachment and good electrical connection between the stem and the block of carbonaceous material.
In operation, an electrolysis plant requires regular replacement of the anodes which are consumed throughout the aluminium production process. Regeneration of a spent anode is an economically necessary operation which involves removing the cooled electrolyte bath attached to the anode butt (residual carbon block), then removing the butt and the thimbles to recycle them, and finally cleaning and if necessary rectifying the assembly made up of the stem and the anode leg in order to associate it with a new block of carbonaceous material and thereby to obtain a new anode. In addition, certain new anodes (typically 1 to 2% of them) may be defective, for example a cracked block of carbon, and must be set aside so that they can be returned and recycled directly with the spent anodes, without ever having been inserted into the electrolysis cells.
Until now, the leftover fragments of anode and the thimbles were removed using stripping machines, the anode being held in vertical position. The anode legs were more or less directly subjected to the forces exerted by the stripping machine and their lifespan was very short.
Generally speaking, the stripping machines are specialised: those used to remove residual blocks are called “anode butt strippers” and those used to remove the thimbles are called “thimble strippers”. Certain devices, called “butt and thimble removal presses” allow anode butt breaking and thimble stripping to be carried out in a single movement.
In practise, owing to the forces generated, the thimble strippers remove one single thimble at a time. For example, German patent application DE 44 10 599 describes a device enabling anode butt and thimble stripping around each stub of an anode leg: a clamp closes to partially rotate said stub, without touching it, its lower face used as an axial stop that prevents the movement of the thimble which is driven, via the stub, by a punch. The stop device and the punch are mobile in relation to the stub and can move towards one other by means of an actuator. By means of a cavity worked into the stop device and which surrounds the stub, with a diameter greater than that of the thimble, the punch initially breaks up the butt, removes the butt fragments and then deforms and detaches the thimbles. Such a thimble stripping device, associated with or without an anode butt stripping device, has the advantage of soliciting the anode stem only locally, the stripping forces being directly supported by the upper part of the grip which acts as a counter brace. However, this particularly bulky device can work on one single stub only, which presents the drawback of requiring a significant amount of time to completely strip an anode stem.
Several documents in the prior art describe less bulky machines working simultaneously on several stubs.
French patent application FR 2 718 989, and U.S. Pat. No. 5,733,423, propose a thimble stripping machine equipped with a plurality of twisting devices acting simultaneously on each thimble. In this solution, the replacement of punches moving axially with devices driven in rotation requires separating anode butt stripping from thimble stripping operation, that is to specifically use a specialised machine for stripping the anode butts then a specialised machine for stripping the thimbles. Furthermore, the risk of damaging the anode legs does not disappear, even if the type is changed: the twisting torque is transmitted to a brittle zone of the stub, where the latter is welded to the transverse branch.
U.S. Pat. No. 5,956,842 describes a combination anode butt and thimble removal apparatus with vertically moving punches associated with axial stops. These are slender punches, placed below the stubs and in line with them, of smaller diameter than that of the stubs and acting upwards. When the punches move up, the later first make contact with the butt then, after the butt is stripped, make contact with the lower face of the stubs, such that the top face of the thimbles are blocked by a stop, the thimbles deform and are detached from the stubs, while remaining trapped by the slender punches. The stop is made up of the lower face of the clamps which close individually around each stub without touching them. The drawback associated with this solution resides in the amount of force that must be provided to simultaneously strip the thimbles from all the stubs. Furthermore, the clamps themselves bear on a common plate, the support of the stripping forces is not really customized such that the anode leg remains sensitive to the unbalanced stresses resulting from the imperfectly symmetrical stresses of the anode leg.
Furthermore, the butt and thimble removal apparatus proposed by the applicant in French patent application FR 2 894 988, presents jaws that close individually around each stub so that a stop prevents the thimbles from rising, driven by the punches via the stubs. The individual support of the stripping forces allows the anode legs to be treated but significant force must also be provided here to simultaneously strip the thimbles from all the stubs.
International application WO01/57291 addresses the well-known problem of the imperfect geometry of anode stems frequently used and recycled in an electrolysis workshop: their transverse arms are more or less inclined and the stubs are more or less worn, such that the ends of the stubs and the associated thimbles are not all on the same level in relation to the forward movement of the punches. According to the author of WO01/57921, the imbalance of forces due to the imperfect routine geometries of the anode legs is such that the movement of the punches and their contact with the end of the stubs must be checked individually: in this way, the moment when each punch enters into contact with the stub with which is associated is controlled and the stripping of the stubs is triggered in a synchronised manner. This device thus requires the implementation of a large set of individual punch actuators and a complex electronic control system for these actuators. Furthermore, a significant amount of force must always be supplied in order to simultaneously strip the thimbles from all the stubs.
The object of the invention is to define a process for stripping thimbles from used (or scrapped new) anodes that does not present the drawbacks presented above, particularly which enables thimbles to be stripped from all stubs of an anode leg in one step and with a single machine, without the use of considerable force, without requiring the introduction of a complex electronic control system, while attempting to keep the risk of damage to the anode leg as low as possible.
Advantageously, the machine used to implement this process could also be used to remove the anode butts. In addition, in this last case, the anode block fragments and thimbles could advantageously be sorted upon exiting this same machine, as in FR 2 894 988, and then sent directly to the anode manufacturing workshops for recycling.